t^3-2t[t^2-2(t-3)]= t^3-2t[t^2-2t+6]=t³-2t³+4t²-12t=-t（t²-4t+12）
t^3-2t[t^2-2(t-3)]=t^3-2t[t^2-2t+6]=t^3-2t^3+4t^2-12t= - t^3+4t^2-12t
